In code division, multiple access (CDMA) cellular telephony, a mobile telephone communicates with a base station where each user and each base station has its own xe2x80x9cspreading codexe2x80x9d used for modulation of the transmitted signal and for separating the different signals corresponding to different users/subscribers. Typically, a receiver picks up many echoes (multipath reflections) of a transmitted signal, each having different and randomly varying delays and amplitudes. In such a scenario, the spreading codes that were originally designed to be orthogonal among the different users are no longer orthogonal and the users are no longer separated. Consequently, a mobile unit, when attempting to detect only a single user, regards all other channel users (including signals from other base stations) as creators of interference. One exemplary single-user multipath detector is a rake receiver that combines the different channel paths into a single replica of the transmitted signal.
The rake receiver detects the signal of interest while treating the interference from other users as background noise. A multiple-user detection (MUD) scheme, on the other hand, incorporates various levels of a-priori information on the interference and may significantly outperform the rake receiver (or any other single-user receiver). The maximal level of a-priori information occurs when the interfering signal is completely known to the receiver, as is the case in U.S. Pat. No. 6,034,986. In the past two decades, numerous methods for multi-user detection were developed based on different levels of utilization of a-priori information and leading to different levels of receiver complexity.
Unfortunately, multi-user detection schemes are significantly more complex than single-user ones. Not only does multi-user detection require (either explicitly or implicitly) processing the received signal with a bank of despreaders (with each despreader being matched to a distinct user), the outputs of this despreader bank must further be processed according to some a priori criterion, such as the maximum likelihood criterion, whose complexity is exponential in the number of users.
Exponential complexity is intolerable unless the number of users is extremely small, which is not the case in cellular communications. Therefore, suboptimal, linear MUD schemes were developed, among them are the decorrelator and/or the minimum mean squared error (MMSE) receiver. Still, linear MUDs require that a signature cross-correlation matrix, or a rescaled version of which, be inverted with each new symbol/block that arrives. This is computationally too heavy to be performed by the mobile unit. Numerous methods for approximating matrix inversion were suggested recently, such as Verdu""s first order approximation (O(K) operations per symbol per user).
Another weakness of existing MUD schemes for long code DS-CDMA (where the spreading code spans more than one symbol) is that there is no simple approach to turn them into the more desirable form of adaptive algorithms, since a symbol rate adaptive algorithm cannot track the symbol rate variations of the cross-correlation matrix. In fact, the problem of adaptive algorithms for long code DS-CDMA is regarded an open problem in the scientific literature.
U.S. Patent Publication WO 00/18030, assigned to the common assignee of the present invention, recognizes that the spreading codes are cyclic and describes a method of processing received signals by dividing the cyclic spreading codes into a plurality of sections. Each section is assigned a different cost function and a respective set of receiver parameters that are used to minimize the associated cost function. Portions of the received signal which were spread by the same section of code are jointly processed, so as to dynamically update the associated set of receiver parameters.